Why do some ads seem to work better on certain days, or why might a medical treatment look effective for one group but not when we look at everyone together? Understanding how two categorical variables—like “day of the week” and “ad clicked” or “treatment group” and “outcome”—relate to each other is a key skill in data analysis. In this chapter, we’ll learn to organize, visualize, and interpret these relationships using simple tables, a few clever charts, and a detective’s eye for patterns that can hide in plain sight.